Known from the state of the art are a large number of measuring devices, which are equipped with two electronic units, which are often galvanically isolated from one another. The primary side electronics unit is connected with the energy supply of the measuring device, while the secondary side electronics unit is associated with the sensor. Provided for energy and data transmission are one or more galvanically isolated interfaces, which can be embodied inductively or capacitively. Communication between the two electronic units occurs, for example, from the primary side to the secondary side via frequency modulation and in the reverse direction via load modulation, such as is described in DE 10 2006 051 900 A1. Also, it is known to use amplitude modulation. Often, no, or only a unidirectional, communication occurs from the primary side to the secondary side via the galvanically isolated interface and an optocoupler serves for data transmission from the secondary side to the primary side.
As a rule, the secondary side transmits to the primary side measurement data, which represent the process variable, while from the primary side to the secondary side, above all, parameter data or sensor-specific, identifying data are transmitted. Based on space, energy and cost, the number of interfaces between primary side and secondary side is, as a rule, reduced to a minimum. In this way, the transmissible information is strongly limited.